
On Fri, 23 Apr 2021, Galaxy Being wrote:
I'm in Bird's Thinking Functionally with Haskell and he has this code to transpose a matrix based on a list of row lists transpose :: [[a]] -> [[a]] transpose [xs] = [[x] | x <- xs] transpose (xs:xss) = zipWith (:) xs (transpose xss)
then he says transpose can be rewritten with this pattern
transpose [] = ...
what could be the rest of it? The answer he gives is
transpose2 :: [[a]] -> [[a]] transpose2 [] = repeat [] transpose2 (xs:xss) = zipWith (:) xs (transpose2 xss)
where repeat [] gives an infinite list of repetitions. And, he says, note that
transpose [xs] = zipWith (;) xs (repeat []) = [[x] | x <- xs]
I suppose I get this last equation, but I don't understand repeat in transpose2. Can someone explain this to me?
With, say transpose [] = [], the (zipWith (:)) would be shortened to the empty list. In contrast to that, (repeat []) provides as many list ends as needed.